1. Field of the Invention
The present invention relates to an image pickup apparatus and a method of controlling the same which allow exposure control modes including a manual exposure control mode to be selected.
2. Description of the Related Art
In an image pickup apparatus having an automatic exposure (hereinafter, referred to as AE) control function, an exposure amount during shooting is automatically determined by a built-in central processing unit (CPU) or the like.
Japanese Patent Laid-Open No. 5-014803 suggests an image pickup apparatus which allows a shooting mode, such as an aperture value (hereinafter, referred to as Av) priority mode and a shutter speed (hereinafter, referred to as Tv) priority mode, to be easily selected.
When an exposure control value determined through the AE control is corrected, or when a user sets a desired control value in the Av priority mode or the Tv priority mode, for example, a control value may be set by a measure of a ½ step or a ⅓ step.
Such a step number for setting the control value is a measure of the control. For example, when an F-number of 5.6 (also, hereinafter expressed like F5.6) is incremented by one step, the F-number may become F4. In a case where the control value is set by a measure of a ⅓ step, when F5.6 is incremented by a ⅓ step, the F-number may become F5.0.
The F-number is an aperture value obtained such that a focal length is divided by an effective opening diameter of the aperture. As the value becomes small, the brightness becomes enhanced.
However, in some cases, the exposure control value may not be corrected by the desired measure. The reasons are herein described below.
Av may be expressed as follows:Av=2 Log 2(f/D)  (1)where D is an effective diameter of the aperture, and f is a focal length.
Also, the following expression is established:Av+ΔAv=2 Log 2(f/(D+ΔD))  (2)where ΔD is an error of an effective diameter of the aperture, and ΔAv is an error of Av.
Based on Expressions 1 and 2, ΔAv is expressed as follows:ΔAv=2 Log 2 (1/(1+ΔD/D))  (3)
As described above, since ΔAv which is the error of Av is determined in accordance with ΔD, as ΔD becomes small, a driving amount of the aperture may cause an error more frequently.
In particular, when the aperture is controlled to an opening side, the error in driving amount of the aperture with respect to the opening diameter becomes relatively small. However, when the aperture is controlled to a closing side, the error in driving amount of the aperture with respect to the opening diameter may become relatively large.
Accordingly, based on Expression 3, the resolution of the aperture control becomes fine (results in high accuracy) as the aperture is controlled to the opening side (large), and the resolution becomes rough (results in low accuracy) as the aperture is controlled to the closing side (small).
FIG. 5 is a graph showing characteristics of step numbers and F-numbers in aperture control using a stepping motor. As shown in FIG. 5, the resolution is sufficient between a step S0 of F2.8 (opening side) and a step S of F4. In contrast, the resolution is insufficient between a step S3 of F8 and a step S4 of F11 (minimum aperture). The resolution is a minimum measure for the steps of a stepping motor so as to move the aperture by one step.
For example, in the case of the aperture control, if the minimum measure for the control step numbers corresponds to the resolution of the control at the closing side, then the control at the opening side with the high resolution may not be utilized.
On the other hand, if the minimum measure corresponds to the resolution at the opening side, then the control may be unstable at the closing side, thereby insufficiently providing the performance of a control device.
Similarly, Tv may be expressed as follows:Tv=Log 2(1/T)  (4)where T is an exposure time.
Also, the following expression is established:Tv+ΔTv=Log 2(1/(T+ΔT))  (5)where ΔT is an error of the shutter speed, and ΔTv is an error of Tv. The error is caused by a delay from when a signal for moving a shutter is output until when the shutter is actually moved, the delay depending on an individual difference.
Based on Expressions 4 and 5, ΔTv is expressed as follows:ΔTv=Log 2(1/(1+ΔT/T))  (6)
As described above, since ΔTv which is the error of Tv is determined in accordance with ΔT, as ΔT becomes small, the Tv control may cause an error more frequently.
In particular, in Tv control at a low speed, the error of the Tv control with respect to the exposure time becomes relatively small. In contrast, in the Tv control at a high speed, the error of the Tv control with respect to the exposure time becomes relatively large.
Accordingly, the resolution of the Tv control becomes rough (results in low accuracy) as the speed increases, and becomes fine (results in high accuracy) as the speed decreases.
As described above, in the case of the Tv control, if the minimum measure for the control step numbers corresponds to the resolution of the Tv control at the high speed, then the Tv control the low speed with the high resolution may not be utilized. On the other hand, if the minimum measure for the control step numbers corresponds to the resolution of the Tv control at the low speed, then the Tv control may be unstable at the high speed, thereby insufficiently providing the performance of the control device. When the user performs the exposure correction on the result provided by the camera through the AE, the exposure correction is made by a predetermined measure such as a ½ step or a ⅓ step.
In addition to the above configuration, a histogram function is provided that displays a luminance distribution of an object.
The user may use the histogram function so as to recognize the luminance distribution of the object, and perform the exposure correction while viewing the distribution.
The exposure correction has been based on the measure of a ½ step or a ⅓ step for instance; however, since the exposure control is becoming highly accurate, the correction measure such as a ½ step or a ⅓ step may be insufficient to provide fine exposure control as intended by the user. Meanwhile, the user may process an shot image to obtain a desired luminance through, e.g., image processing using a computer.
FIG. 12 is a graph showing a relationship between an output of the image pickup device and a luminance distribution of an object. In FIG. 12, when the luminance of the object is within “a luminance distribution range 1”, the luminance is within a dynamic range of the image pickup device. In this state, even though the exposure is corrected with a relatively discrete measure such as a ½ step or a ⅓ step, the processing of the luminance using a computer or the like after the image is shot may provide an image in a suitable exposure state that is intended by a person who shot the image but without underexposure or overexposure.
When the luminance of the object is within “a luminance distribution range 2”, the luminance exceeds the dynamic range of the image pickup device. In order to obtain an image in the suitable exposure state, it is important to perform fine exposure correction to adjust the luminance distribution within the dynamic range.
However, if the exposure is corrected with the relatively discrete measure such as a ½ step or a ⅓ step, a correction amount becomes too large, and therefore, an image in the suitable exposure state that is intended by a person who shot the image may not be provided.